cabal2nix.cabal: bumo version number to 1.31
cabal2nix.cabal: bumo version number to 1.31

Fuuzetsu/cabal2nix,jb55/cabal2nix,spencerjanssen/cabal2nix,dmalikov/cabal2nix,bennofs/cabal2nix,gridaphobe/cabal2nix,psibi/cabal2nix